The present invention relates to a machine for producing a fiber web, particularly a paper web, and particularly relates to the dry end of the machine. The machine has a press section followed in the web path by a dryer section. The dryer section comprises a plurality of separated dryer groups, each operable at a respective different speed. Each dryer group includes a plurality of dryers, a plurality of web path reversal rolls, one between each two dryers, and an endless loop web support belt, which is sometimes a dryer felt, passing around the dryer group in a meander path past the dryer cylinders and the reversal rolls and past guide rolls which guide the support belt in the endless loop. The pocket between the dryer cylinders may have a differential pressure box within it. After the web to be produced from a fiber suspension is formed and partly dewatered in a wire end or forming end or wet end of the machine, the web is dewatered mechanically as far as possible in a press. Then the web is dried in a dry end comprised of heatable drying cylinders.
In the prior art single tier dryer sections, the web is restrained while travelling around the dryer cylinder by having a felt press the web against the dryer cylinder. Between dryer cylinders, a vacuum roll was located to restrain the web against the felt during passage around the vacuum roll and thereby restraining the web against cross-machine directional shrinkage. However, the web is not restrained between the place where the web leaves the dryer cylinder and the place where the web is picked up by the vacuum roll.
In order to increase the runnability, it is known to keep the web as reliably as possible on the web support belt at the place where the web runs off from each individual drying cylinder to the reversing roll, and on the straight travel path from the reversing roll to the following drying cylinder. In this respect, the initial region of the dry end presents a particular problem because the paper web is still relatively wet there and it has a tendency to adhere to the wall of the drying cylinder and to detach itself temporarily from the support belt as the web leaves each dryer cylinder. In other words, a so called bubble is formed here between the web of paper and the support belt. In order to reduce the danger of the web of paper tearing, it is attempted to keep the bubble as small as possible. For this purpose, it is known to form a vacuum zone at the run-off place, shown in U.S. Pat. No. 4,359,828, FIG. 3. Another known measure to reduce the danger of the web of paper tearing consists of reducing the distance between the drying cylinder and the adjacent reversing roll as much as possible, shown in International Application WO 83/00514, FIG. 2, or U.S. Pat. No. 4,905,379, FIG. 1.